Improvement in Performance of a Microchannel Plate Detector Assembly Using a Miniature Anode (Micro-node)
Small devices that can rapidly and accurately analyze chemical and biological molecules are required by many industries. Homeland security and the medical community lead the list. Current instruments are either too large to be used as a first responder in the field, too slow in giving results to analyze large amounts of protein data, or too costly for widespread use.
The Johns Hopkins University Applied Physics Laboratory has invented and patented an improved version of commercially available coaxial channelplate detectors have relied upon a disk-shaped metallic center-hole anode to collect the pulse of electrons generated by the microchannel plates. This anode matches the diameter of the channelplates thereby, in theory, maximizing the electron collection efficiency. However, the center-hole anode also creates an extraneous capacitance and significant impedance mismatch between the detector and the signal collection device (i.e. digital oscilloscope). A simple alteration of the anode is found to substantially improve the overall performance of the detector. The rational for this modification relies on the fact that the collection surface of the anode need not match the shape of the channelplates. That is, the ground potential anode will attract electrons emerging from the channelplates regardless of its size or geometry. It is found that a polished pin anode, even if its location is offset from the center of the detector, provides a substantial improvement in signal quality. 1)The pin (or shaped “micro-anode”) presents no impedance mismatch between the 50-ohm cable and the electron collection surface, thereby virtually eliminating the ringing problem. 2)The single ion pulse width is reduced to less than 500 ps/pulse due to the reduction in anode capacitance. 3)The reduction of anode size significantly reduces the “antenna” properties of the detector, resulting in far less extraneous noise and hence much quieter background signals.
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